Method of manufacturing a member of a wheelchair frame, tool for use in such a method and wheelchair frame

Description

The invention relates to a method of manufacturing a member of a wheelchair frame, including: providing a tubular member; bending the tubular member into a desired shape; and adjusting a cross-sectional shape of at least a section of the tubular member.

The invention also relates to a wheelchair frame, including left and right tubular frame members for supporting a seat, each including a curved section.

The invention also relates to a tool for use in a method of manufacturing a member of a wheelchair frame.

The invention also relates to a wheelchair.

An example of such a wheelchair is known. WO 93/08782 discloses a wheelchair formed of a chassis that includes a pair of side frame assemblies connected to each other to form a rigid frame by a seat pan and a cross beam. Each side frame assembly includes three members: a bottom member, a foot rest member and a seat member. Furthermore, each bottom member includes a forward end that extends in a generally horizontal direction, and a rearward end that extends in a generally upward direction. The seat member extends from an opposite end

of the foot rest member to the rearward end of the bottom member. In an embodiment, the seat member and foot rest member can be merged into a single member. The cross-section of the forward end of the bottom member can have any non-round shape. The tubular members of the wheelchair frame can assume various non-circular cross-sectional shapes which par- ticularly facilitate clamping of various lug members to the intermediate portion of a non- circular tubular member, whereby rotation around the tubular member is prevented in order to maintain the desired alignment of the member clamped on the noncircular tubular member. The upper tubular members corresponding to the foot rest members and seat members are generally circular in cross-section. The known wheelchair frame also has casters, attached to opposite ends of a cross bar by means of a clamp. The cross bar has a U-shape, such that the cross bar curves inwardly in a direction towards the rear of the wheelchair when the caster assembly is mounted on the chassis. In a preferred embodiment, the cross bar has a non- round cross-section.

A problem of providing the upper tubular members corresponding to the foot rest members with a circular cross-section is that a closed side frame is required for sufficient rigidity. Otherwise, the foot rest member would be able to swing due to insufficient rigidity of the bent section of the foot rest member.

Conventionally, wheelchair frame members with non-circular cross-sectional shapes are produced by providing a straight piece of tubing with the non-circular cross-sectional shape and then bending it into a desired shape.

A problem of such a conventional method is that one cannot achieve tight bends without reducing the diameter of the tubing.

It is an object of the invention to provide a method and wheelchair frame of the types mentioned above in the opening paragraphs that are suitable for manufacturing a relatively light yet rigid wheelchair, and particularly a method suitable for providing a compact wheelchair.

This object is achieved by the method according to the invention, which is characterised in that at least one step of adjusting the cross-sectional shape is carried out after the step of bending the tubular member into the desired shape.

In an embodiment, the cross-sectional shape is adjusted to a non-circular cross-sectional shape.

The non-circular cross-sectional shape may be a round, in particular oval, shape. In an embodiment of the method, the cross-sectional shape of at least a bent section of the bent tubular member is adjusted.

In an embodiment of the method, the tubular member is formed into a tubular frame member for supporting a seat, comprising a first leg for supporting the seat, a second leg, depending downwardly, in use, and a curved section, forming a transition between the first leg and the second leg.

In an embodiment, the desired shape of the wheelchair frame member includes a curved section bent in multiple planes.

In an embodiment, the desired shape of the wheelchair frame member includes a curved section having in at least one plane a radius of curvature equal to or smaller than four times, more specifically equal to or smaller than three times, a particular cross-sectional diameter, the particular cross-sectional diameter corresponding to one of: a) a diameter of a corresponding bent section of the bent tubular member prior to adjustment of the cross-sectional shape; b) a maximum diameter of the curved section; and c) a minimum diameter of the curved section.

In an embodiment, the step of adjusting the cross-sectional shape includes placing the bent tubular member against a tool for supporting the bent tubular member in the desired shape and exerting a pressing force on the bent tubular member.

The tool may be provided with at least one abutment at at least one position along a length of the bent tubular member for determining the diameter of the adjusted cross-sectional shape in a direction transverse to the pressing force.

According to another aspect, the wheelchair frame according to the invention is characterised in that at least the curved section has a non-circular cross-sectional shape.

In an embodiment of the wheelchair frame, the non-circular cross-sectional shape is a round, in particular, oval shape.

The curved section may be a bent section.

In an embodiment of the wheelchair frame, each tubular frame member includes a first leg for supporting the seat and a second leg, depending downwardly, in use, the curved section forming a transition between the first leg and the second leg.

In an embodiment of the wheelchair frame, the curved section is curved such that the first leg lies in a first plane, parallel to the second leg, and the second leg lies in a second plane, parallel to the first leg, and the first and second planes are at a distance to each other.

An embodiment of the wheelchair frame includes left and right front wheel supports, each connected to a second leg of a respective tubular frame member, for carrying a respective front wheel assembly, wherein the wheelchair frame is configured such that, in use, a wheelchair comprising the wheelchair frame is supported by the front wheel assemblies via only the second of the first and second legs of the tubular frame members.

According to another aspect of the invention, the tool for use in a method according to the invention for adjusting a cross-sectional shape of at least a section of the bent tubular member comprises at least one support structure for supporting the bent tubular member, each support structure conforming to the desired shape of the wheelchair frame member.

In an embodiment, the tool includes at least one part for exerting a pressing force on at least a bent section of the bent tubular member and conforming to the desired shape of the wheel- chair frame member.

An embodiment of the tool further includes at least one abutment at at least one position along a length of the support structure(s) for supporting the bent tubular member, each abutment configured to limit the diameter of the adjusted cross-sectional shape in a direction transverse to the pressing force when the pressing force is applied.

According to another aspect, the wheelchair according to the invention includes a wheelchair frame according to the invention and/or a wheelchair frame comprising a member obtainable by means of a method according to the invention.

The method according to the invention is based on the surprising insight that a particular order of steps in the shaping of the tubular member will allow one to achieve a lightweight, rigid and compact wheelchair. Tubular members are stronger and lighter than members with other profiles, e.g. solid or U-shaped members. Because at least one step of adjusting the cross-sectional shape, which may be the only such step, is carried out after the step of bending the tubular member into the desired shape, the step of bending the tubular member into a desired shape can be carried out whilst the tubular member still has a cross-sectional shape that is suitable for bending with a small radius of curvature. Thus, the method is suitable for providing a relatively compact wheelchair. The adjustment of the cross-sectional shape allows the member to have a non-circular cross-sectional shape in the final state. This allows for an increase in diameter in a particular plane in which, when the wheelchair is in use, bending forces will occur, for example. It is also useful if the tubular frame member is to be bonded (glued, welded) at an angle to a longitudinal axis of a further wheelchair frame member with a relatively small diameter, because the contact area for forming the bond is increased.

If the non-circular cross-sectional shape is a round, in particular, oval shape, then the adjustment in shape can be carried out relatively easily, starting from bent tubing with a generally circular cross-sectional shape. In particular, a pressing force applied unevenly around the circumference of at least the section of the tubular member will result in a round non-circular shape. An oval shape requires a pressing force in only one direction. Thus, this is a particularly easy way of arriving at a cross-sectional shape with a large diameter along a major axis and a smaller diameter along a minor axis. However, it is not the only way. An alternative manner of adjustment of the cross-sectional shape involves hydroforming. In such a step, special fluids are injected into the tubes using high pressure (-1000 bar) fluids. Contour and

size are formed following the pattern of a mould. The result can be a tube with longitudinally varying wall-thickness, as well as a non-circular varying or constant cross-sectional shape. Of course, several adjustment steps can be used in combination, even one before bending and one after bending of the tubular member into the desired shape.

If the cross-sectional shape of at least a bent section of the tubular member is adjusted, the advantages of the method are exploited fully, since one can achieve a smaller radius of curvature of the bent section and still strengthen the bend in a particular direction, particularly in the plane of curvature. Thus, the bent section is made relatively rigid, so that the shape of the bend is maintained, in use. If one were to adjust the cross-sectional shape to e.g. a non- circular shape and then bend the tubular member, one could not achieve a very small radius of curvature, because the tubular member would be at risk of tearing.

If the tubular member is bent into a tubular frame member for supporting a seat, comprising a first leg for supporting the seat, a second leg, depending downwardly, in use and a curved section forming a transition between the first and second leg, then a particularly stiff open frame is achieved. The second leg will, directly or via members of a foot support assembly, support the feet of the user. It may also be connected to a caster wheel assembly, e.g. via wheel support members. Thus, the second leg, downwardly depending, in use, will be subjected to vertical forces that will tend to "unbend" the tubular frame member. By providing it, in particular the bent section, with a non-circular cross-sectional shape, the bend can be stiff- ened. This means that, even though the frame is open, it is still relatively rigid.

If the tubular member is provided with a curved section bent in multiple planes, then more complicated frame shapes can be provided without resorting to separate frame members that have to be joined together, e.g. welded. This makes the frame easier to assemble. It also means that there are fewer potential points of weakness.

If the tubular member is provided with a curved section bent in at least one plane with a radius of curvature equal to or smaller than four times a maximum or minimum cross-sectional diameter of that section, in particular equal to or smaller than three times this cross-sectional diameter - or equal to or smaller than four or even three times the diameter of the cross- sectional shape before adjustment thereof - then the advantages of the method are exploited

more fully. In particular, such small radii are achievable using a tubular member with a circular cross-sectional shape, and thus a small diameter. Subsequently, the diameter is increased in at least one direction to a value that would not be achievable if at least the final adjustment of the cross-sectional shape had been carried out prior to bending.

If the step of adjusting the cross-sectional shape includes placing the bent tubular member in a tool for supporting the bent tubular member in the desired shape, and exerting a pressing force on the tubular member, then it is ensured that the desired shape is maintained relatively well during the adjustment of the cross-sectional shape.

Where the tool is provided with at least one abutment at at least one position along a length of the bent tubular member for determining the diameter of the adjusted cross-sectional shape in a direction transverse to the pressing force, the cross-sectional shape, in particular the maximum diameter, can be controlled relatively accurately. It is thus possible to manufacture the frame member to the specifications provided to ensure adequate stiffness.

The wheelchair frame according to the invention is based on the surprising insight that it is possible to provide a tubular member for supporting a seat with a curved section with a non- circular cross-sectional shape to make the curved section stiffer if one abandons the traditional way of manufacturing frame parts, which is by bending members with the final intended cross-sectional profile into the desired shape. The traditional way can only succeed if the curved sections have a relatively large radius of curvature, which is an obstacle to providing a compact wheelchair. By providing the left and right tubular frame members for supporting a seat with a curved section having a non-circular cross-sectional shape, it is possible to make the curved section rigid enough that it is not necessary to resort to a closed side frame. This results in an even more compact and lightweight wheelchair.

If the non-circular cross-sectional shape is a round, in particular, oval shape, then the frame member has no edges that might cause damage or injury. Moreover, it is possible to clamp other wheelchair components, e.g. axle plates or accessories such as arm rests, to the frame member relatively easily. The non-circular shape, however, prevents rotation of the clamp around the tubular frame member. An oval shape is relatively easy to manufacture, starting from tubing with a generally circular cross-sectional shape.

In an embodiment, the curved section is a bent section. This makes the frame suitable for manufacture by bending tubing into the desired shapes of the frame members. It can therefore be made of metal or metal alloys, rather than composite components that have to be moulded. This has the advantage of reducing costs. It also makes it economically feasible to manufac- ture frame members in a range of sizes.

In an embodiment, the tubular frame members each include a first leg for supporting the seat and a second leg, depending downwardly, in use, the curved section forming a transition between the first and second leg.

The second leg will, directly or via members of a foot support assembly, support the feet of the user. It may also be connected to a caster wheel assembly, e.g. via wheel support members. The non-circular cross-sectional shape strengthens the tubular frame members in at least one plane of curvature, making the bend more rigid. Particularly where the second leg and wheel support members are the only connection between the frame and caster wheel assembly, this rigidity is desirable to provide a lighter-running wheelchair.

If the curved section is curved such that the first leg lies in a first plane, parallel to the second leg, and the second leg lies in a second plane, parallel to the first leg, and the first and second planes are at a distance to each other, then the effect is to narrow the distance between the second legs of left and right tubular frame members relative to the distance between the first legs of the left and right tubular frame members. Since the first legs support the seat, the seat can be relatively wide. By contrast, the second legs can provide support for the wheelchair occupant's legs, because they are relatively close together. The front end of the wheelchair is in any case more compact.

If the wheelchair frame includes left and right front wheel supports, each connected to a second leg of a respective tubular frame member, for carrying a respective front wheel assembly, and the wheelchair frame is configured such that, in use, the wheelchair is supported by the front wheel assemblies via only the second of the first and second legs of the tubular frame members, then the wheelchair has generally L-shaped, or open, side frames, reducing weight.

According to another aspect of the invention, the tool for use in a method according to the invention for adjusting a cross-sectional shape of at least a section of the tubular member comprises at least one support structure for supporting the tubular member, the support structure conforming to the desired shape. This tool is thus suitable for ensuring that the desired shape is maintained whilst the cross-sectional shape of at least sections of the tubular frame member are adjusted.

If the tool includes at least one part for exerting a pressing force on at least a bent section of the tubular member and conforming to the desired shape of the tubular member, then pinching of the tubular frame member at certain locations along its length by the tool is largely pre- vented. There are thus fewer potential weak spots, and the frame can be relatively smooth.

If the tool further includes at least one abutment at at least one position along a length of the support structure for supporting the tubular member, each abutment positioned to limit the diameter of the adjusted cross-sectional shape in a direction transverse to the pressing force when the pressing force is applied, then it is possible to provide a non-circular cross-sectional shape with a maximum diameter within a narrow tolerance range of an intended value.

The invention will be explained in further detail with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of a wheelchair, with the seat and backrest adjustment mechanism removed for clarity;

Fig. 2 is a perspective view of frame members, including parts of a backrest frame, of the wheelchair of Fig. 1;

Fig. 3 is a perspective view of a tubular member provided as starting product for a method of manufacturing a side frame member of the wheelchair frame;

Fig. 4 is an end view of the tubular member of Fig. 3;

Fig. 5 is a side view of the tubular member of Figs. 3 and 4;

Fig. 6 is a perspective of view of the tubular member after bending in the desired shape;

Fig. 7 is an end view of one end of the tubular member at the stage in which it is shown in Fig. 6;

Fig. 8 is a side view of the tubular member at the stage in which it is shown in Figs. 7 and 8;

Fig. 9 is a perspective view of the tubular member after adjustment of its cross- sectional shape;

Fig. 10 is an end view of one end of the tubular member at the stage in which it is shown in Fig. 9;

Fig. 11 is a side view of the tubular member at the stage in which it is shown in Figs. 9 and 10;

Fig. 12 is a bottom perspective view of a first component of a tool for transforming the tubular member from the stage at which it is shown in Figs. 6-8 to the stage in which it is shown in Figs. 9-11;

Fig. 13 is a top perspective view of a second component of the tool of which the first component is shown in Fig. 12; and

Fig. 14 is a perspective view of the tool of Figs. 12 and 13, in use.

Referring to Fig. 1 , a wheelchair 1 shown here by way of example is supported by left and right rear main wheels 2,3 and left and right caster wheel assemblies 4,5, comprising caster wheels 6,7. The wheelchair 1 comprises a base frame and backrest frame 8.

The base frame comprises left and right side frame members 9,10, arranged on opposite sides of a central axis (not shown) aligned with a direction d of forward displacement of the wheelchair 1. The left and right side frame members 9,10 are generally L-shaped. In other words, the wheelchair 1 has an open frame. The side frame members 9,10 each terminate longitudi- nally in a first leg 11,12 and a second leg 13,14. Each first leg 11,12 transitions into a second leg 13,14 via a respective curved section 15,16. The first legs 11,12 extend generally horizontally, and the second legs 13,14 depend downwardly. It is noted that the angle of the first legs 11,12 to the horizontal can be adjusted by adjusting the rear height of the first legs 11,12 above ground. They are thus only predominantly oriented in a horizontal direction, not exclu- sively.

The first legs 11,12 are arranged for supporting a seat (not shown) of the wheelchair 1. In particularly, a seat sling (not shown) can be slung between the first legs 11,12 of the side frame members 9,10, on top of which a seat cushion (not shown) of any shape or configuration can be placed.

The curved sections 15,16 of the side frame members 9,10 are curved in multiple planes, such that the second legs 13,14 are separated from each other by a shorter distance than the first legs 11,12 of the side frame members 9,10. Thus, the seat can be relatively wide, whereas the second legs 13,14 of the side frame members 9,10 provide support for the wheelchair occupant's legs.

The second legs 13,14 are supported by the caster wheel assemblies 4,5 via caster struts 17,18 in which the caster wheel assemblies 4,5 are partially accommodated. The caster struts 17,18 place the caster wheels 6,7 at a wider distance from the longitudinal central axis of the wheelchair 1, to provide stability. They are also attached to second legs 13,14 of the side frame members 9,10 at an angle thereto so as to be oriented at least partly in a direction opposite to the direction d of displacement of the wheelchair 1, i.e. closer to the rear wheels 2,3. This makes the manoeuvrability of the wheelchair 1 relatively good, by shortening the wheelbase. Because the wheelchair 1 has an open frame, it is supported by the caster wheels 6,7 only via the caster struts 17,18 and second legs 13,14. There is no other connection between the caster wheel assemblies 4,5 and the first legs 11,12 of the side frame members 9,10. Similarly, the

caster struts 17,18 form the only connections between the caster wheel assemblies 4,5 and the side frame members 9,10.

It is desirable to place the rear wheels 2,3 as close together as possible, in particular also as close to the side frame members 9,10 as possible. The seat should be wide enough to ac- commodate an occupant comfortably, but, overall, the wheelchair 1 should be narrow for better manoeuvrability and access to buildings, transport means and the like. This is only possible if the set camber of the rear wheels 2,3 is maintained when the wheelchair 1 is occupied. There must be no "sagging" of the frame, in particular rotation or torsion of the side frame members 9,10, under the weight of the occupant. Otherwise, the wheels 2,3 would angle in- wards at the top towards the first legs 11,12, and run against them or against side guards (not shown) mounted to the first legs 11,12 of the side frame members 9,10. To prevent this, the first legs 11,12 are directly connected by a cross-brace 19 (see also Fig. 2), and indirectly by an axle tube 20. Additionally, a foot rest assembly with left and right foot rest frame members 21,22, in general alignment with and connected to the second legs 13,14 includes at least one cross-member 23 interconnecting the left and right foot rest frame members 21,22. A further cross-member (not clearly visible in Fig. 1) is situated below a foot plate 24.

For adjustment of the height of the foot plate 24, it is contemplated that the left and right foot rest frame members 21 ,22 of the foot rest assembly be accommodated within the second legs 13,14 in telescoping manner. Means for arresting telescopic movement of the left and right foot rest frame members 21,22 in one of a number of positions will generally be provided, e.g. in the form of biased pins in the foot rest frame members 21,22, arranged to cooperate with any of a series of holes in the second legs 13,14 of the side frame members 9,10.

The first legs 11,12 of the side frame members 9,10 are each linked to the axle tube 20 by a respective axle plate 25,26 to which respective axle tube clamps 27,28 are connected in one of a number of pre-determined positions on the axle plate 25,26. The interconnection between the axle tube clamps 27,28 and the axle plates 25,26 allows for movement of the axle tube 20 between the different pre-determined positions at varying distances to the first legs 11,12 of the side frame members 9,10. The positions are at varying distances to the seat. In this manner, the rear seat height can be adjusted, because the axle tube 20 accommodates camber tubes (not shown in detail) for holding axles of the rear wheels 2,3.

It is desirable to make the curved sections 15,16 as stiff as possible, particularly in the plane of the downward curve. The propulsion force exerted by the user is transmitted in generally horizontal direction along the first legs 11,12, whereas roll resistance forces on the caster wheels 6,7 are in the opposite direction and amplified by the second legs 13,14, which act as levers, so that a bending moment will be exerted on the curved sections 15,16. Due to the absence of an interconnection between the axle tube 20 and the caster struts 17,18, there is no counteracting force to the resistance forces, other than the elastic force exerted via the second legs 13,14. If the curved sections 15,16 are therefore not stiff enough, a lot of the wheelchair occupant's energy will be wasted through deformation of the left and right side frame mem- bers 9,10. Moreover, there is also a vertical force exerted by the caster struts 17,18 via the second legs 13,14 on the curved sections 15,16, which force is again amplified, because the second legs 13,14, which are oriented in a direction that is partially parallel to the direction d of displacement, also act as levers for these forces.

It is not desirable to make the curved sections 15,16 heavier, because this would partially ne- gate the advantage of an open frame. Therefore, the curved sections 15,16 are provided with a non-circular cross-sectional shape, with a relatively large cross-sectional diameter in a plane of at least the downward curve. The manner in which the left and right side frame members 9,10 are produced allows this cross-sectional shape to be provided by bending of a tubular member without having to provide the curved sections 15,16 with a very large radius of curvature. This means that the length of the wheelchair 1 (its dimensions in the direction parallel to the direction d of displacement) is relatively small.

Steps in the production of the left side frame member 9 are illustrated in Figs. 3-11. In a first step (Figs. 3-5) a generally straight tubular member 29 that is circle-cylindrical in shape is provided. The straight tubular member 29 is made of metal or a metal alloy, so that the left side frame member 9 can later be bonded easily to the cross-brace 19, e.g. by welding. Suitable materials include aluminium-scandium alloys, aluminium alloys from the 7000 series, particularly aluminium 7003, or aluminium from the 6000 series. Aluminium 7000 has a relatively high tensile strength, which allows one to bend the straight tubular member 29 into a shape with curved sections having small radii of curvature. The straight tubular member 29

has a first diameter Di. Di has a value in the order of centimetres, specifically, in a range between 10 mm and 50 mm, more particularly 3 cm.

The straight tubular member 29 is optionally butted before the next stage, i.e. provided with a longitudinally varying wall thickness.

In a next step, the straight tubular member 29 is bent into a desired shape, resulting in a bent tubular member 30 with a generally circular cross-sectional shape (Figs. 6-8). That is to say, the original cross-sectional shape is not changed prior to bending, so that the diameter of the bent tubular member 30 corresponds generally to the first diameter D ₁ . Fig. 7 shows that the bent tubular member 30 includes a first leg 31, a second leg 32 and a curved section 33 bent in at least two planes. The first leg 31 lies in a first plane, parallel to the second leg 32. The second leg 32 lies in a second plane, parallel to the first leg 31, and the first and second planes are at a distance x to each other.

In a subsequent step, the bent tubular member 30 is subjected to an adjustment of the cross- sectional shape of at least a longitudinal section thereof, to arrive at the side frame member 9. In the present example the cross-sectional shape is adjusted to a generally oval shape (cf. Fig. 10) along the entire length of the side frame member 9. The oval cross-sectional shape has a maximum diameter a in a direction in a plane that is parallel to both the first and second leg 11,13, which is the plane of downward curvature. It has a minimum diameter b in a transverse direction. The maximum diameter a is larger than the first diameter D ₁ . The minimum diameter b is smaller than the first diameter Dj .

The curved section 15 has a centre line with a radius R of curvature, at least in the plane of downward curvature, that is equal to or smaller than four times the maximum cross-sectional diameter a, in particular equal to or smaller than three times the maximum cross-sectional diameter a. In fact, in the contemplated embodiment, it is equal to or smaller than four times, even three times, the first diameter Di. In certain embodiments, with appropriate processing conditions, which may include heat-treatment, the radius R of curvature can be equal to or smaller than four times or even three times, the minimum diameter b. Such a small value of the radius R of curvature is more easily achievable by adjustment of the cross-sectional shape after bending, rather than before.

Figs. 12-14 show a tool for carrying out the ovalising step. The tool comprises a first tool component (Fig. 12), referred to herein as upper tool component 34, and a second tool component (Fig. 13), referred to herein as lower tool component 35.

The lower tool component 35 includes supports 36-38 that conform to the shape of the bent tubular member 30, so that it is supported essentially along the entire length of the section(s) to be ovalised. In this example, the whole bent tubular member 30 is supported to one side. A further support 39 is provided for production of the other one of the left and right side frame members 9,10. By rearranging the supports 36-39, the right side frame member 10, which is a mirror image of the left side frame member 9, can be provided. Thus, the lower tool component 35 is modular, having separate supports 36-39 placed on a frame so as to permit reconfiguration of the positions and orientations of the supports 36-39. The lower tool component 35 and supports 36-39 are therefore usable to produce more than one type of tubular frame member.

In the ovalising step, a pressing force is applied through the upper tool component 34. The upper tool component 34 also conforms to the desired shape of the side frame member 9, in that it comprises a profiled component 40 of the appropriate shape. The profiled component 40 within the upper tool component 34 can be replaced by one that is its mirror image, in order to manufacture the other of the left and right side frame members 9,10.

Guides, in the form of pins 41a-f are provided in the upper tool component 34, which co- operate with sockets 42a-f, or similar guide components, provided on the lower tool component 35 to position the upper and lower tool components 34,35 with respect to one another. The pressing force exerted by a hydraulic or electrical actuator (not shown) move the upper and lower tool components 34,35 towards each other. Adjustable stops 43a-f limit expansion of the bent tubular member 30 in the planes of support defined by the supports 36-39. These in effect determine the maximum diameter a of the oval cross-sectional shape that is achieved.

The invention is not limited to the embodiments described above, but can be varied within the scope of the accompanying claims. Features of the invention as disclosed in the description,

claims and drawings may be essential to its implementation individually or in any combination.

Embodiments of the method are conceivable in which certain sections of the side frame members 9,10 retain a circular cross-sectional shape, e.g. because they are intended as parts of a telescopic assembly. Such a varying cross-sectional shape can be achieved by using appropriately shaped supports 36-39 and an appropriately shaped upper tool component 34, for example. In the method, at least one step of adjusting the cross-sectional shape is carried out after the step of bending the tubular member into the desired shape. In the method disclosed above in detail, all adjustments from a circle-cylindrical cross-sectional shape are carried out after the desired shape of the tubular frame member has been arrived at.

LIST OF REFERENCE NUMERALS Wheelchair Left rear wheel Right rear wheel Left caster wheel assembly Right caster wheel assembly Left caster wheel Right caster wheel Backrest frame Left side frame member Right side frame member Left first leg Right first leg Left second leg . Right second leg Left curved section Right curved section Left caster strut Right caster strut Cross-brace Axle tube Left foot rest frame member Right foot rest frame member Foot rest cross-member Foot plate Left axle plate Right axle plate Left tube clamp Right tube clamp Straight tubular member Bent tubular member

First leg of bent tubular member

Second leg of bent tubular member

Curved section of bent tubular member

Upper tool component

Lower tool component

1 ^st Support

2 ^nd Support

3 ^rd Support

4 ^th Support

Profiled component of upper tool componenta-f Pins a-f Sockets a-f Stops